At the present time in the production of aluminium, it is customary to evaporate aluminate liquors in multiple effect evaporator systems consisting of a set of evaporators arranged in series. Said evaporators are intended to concentrate liquors or to deposit in part solid fractions from these liquors, the liquid fraction being drawn off as a vapour due to heating the liquor with a heating medium. In a multiple effect evaporator system, the evaporators are connected with each other by means of a pipeline and vapour pipeline system so that the heating medium and the superheated liquor to be evaporated successively pass through all the evaporators. In the evaporators, steam is generally used as a heating medium.
The scheme of connection of evaporators is referred to as a direct-flow one when the flow direction of the heating medium and that of the liquor to be evaporated coincide, whereas the scheme of connection of evaporators in such a system is referred to as a counter-flow one when the flow directions of the heating medium and the liquor to be evaporated from one evaporator to another are oppositely directed. A mixed connection of evaporators in an evaporator system is also possible, the direct-flow and counter-flow schemes of connection of the evaporators being combined in such a system.
Known in the art are evaporators having forced circulation of the liquor to be evaporated, comprising upper and lower pulp chambers and a heating chamber disposed there-between. Mounted in the heating chamber are vertical tube members heated with a heating medium. Liquor to be evaporated is pumped via a distribution device to the top open ends of the tube members and runs down inside these members and starts boiling due to heat absorption from the heating medium. The two phases therewith being separated, namely vapour and the evaporated liquor of a higher concentration which is substantially a pulp. The vapour evolved on boiling of the liquor is discharged from the inside of the evaporator via a vapour pipeline and the pulp is pumped from the lower pulp chamber into the upper pulp chamber of the same evaporator.
Among the disadvantage of forced circulation evaporators is their intricate construction conditioned by the use of the distribution device and pump.
Evaporators with natural circulation of the liquor to be evaporated are of more simple design, wherein the insides of the upper and lower pulp chambers are connected via outside circulation tubes arranged on both sides of the evaporator shell. In these evaporators, similarly to evaporators with forced circulation of the liquor to be evaporated, said liquor enters inside the vertical tube members of the heating chamber and starts boiling there, and a vapour column is formed in the top portion of these members. Thus, a difference in height occurs between the columns of the liquor to be evaporated in the outside circulation tubes of the evaporator and the columns of the liquor to be evaporated in the vertical tube members of the heating chamber, which results in a circulation head ensuring the natural circulation of the liquor to be evaporated between the upper and lower pulp chambers through the vertical tube members of the heating chamber and through the outside circulation tubes.
Among the disadvantages of evaporators having the boiling zone located inside vertical tube members of the heating chamber is the fact that the boiling of the liquor to be evaporated is accompanied with separation and deposition of the solid phase (salts) on the inner surface of the vertical tube members, i.e. incrustation of these members around the cross-section occurs. This results in decreasing coefficient of heat transfer from the heating medium through the walls of the vertical tube members to the liquor to be evaporated circulating therethrough, thereby decreasing the output of the evaporator. Besides, this entails frequent washings of the evaporator which is to be laid up for the washing period, which, in turn, exerts a detrimental effect on the output of the evaporator.
Said disadvantages are inherent in both groups of evaporators having the boiling zone located inside of the tube members, namely the evaporators with natural circulation of the liquor to be evaporated and the evaporators with forced circulation of such a liquor.
Also known in the art is an evaporator with the boiling zone external with respect to the vertical tube members of the heating chamber (cf., for example, GOST 11987--66, M., 1974, type I, version 5).
The evaporator comprises a shell and upper and lower pulp chambers, and a heating chamber disposed therebetween. All said three chambers are formed by two transverse tube sheets rigidly secured on the innerside of the evaporator shell. Rigidly connected with their ends to the transverse tube sheets are vertical tube members. Disposed in the upper pulp chamber, coaxially to said chambers, is a boiling chamber of the liquor to be evaporated, shaped as a cylindrical sleeve mounted on the central portion of the upper transverse tube sheet. The evaporator is also provided with connections for feeding the liquor to be evaporated and the heating medium into the evaporator, connections for removal of vapour and pulp from this evaporator, and also connections for removal of non-condensable gases and condensate of the spent heating medium.
In the prior art evaporator described, the boiling chamber of the liquor to be evaporated mounted on the central portion of the upper transverse tube sheet over the vertical tube members has a rather considerable height. Hence, liquid column located in the boiling chamber over the vertical tube members of the heating chamber exerts a considerable pressure upon lower layers of the liquor to be evaporated, contained in the vertical tube members, which prevents the liquor to be evaporated from boiling within these members. As for boiling, it occurs in the top portion of the boiling chamber. A disadvantage of the evaporator described consists in incrustation of the vertical tube members around their cross-section due to deposition of the solid phase on the inside of said members, which solid phase is separated during boiling and circulation of the liquor to be evaporated. Even though this shortcoming inherent in the evaporator design having the boiling zone external to the vertical tube members is much lesser as compared to the evaporators described above, wherein the boiling zone is located inside of the vertical tube members, yet it still exists.
Another disadvantage of the prior art evaporator described consists in a rather low intensity of the liquor evaporating process since a great deal of energy of the circulating liquor is wasted to overcome the resistance in the hydroulic circuit of the evaporator. As a consequence, the evaporator described can be sufficiently usable only in a multiple effect evaporator system operating the direct-flow scheme, wherein the energy of the superheated evaporated liquor is sufficient to ensure an intensive circulation of the pulp. However, when employing a couter-flow multiple effect evaporator system, wherein said evaporator is the first body in the direction of the heating medium and the last one in the direction of the liquor to be evaporated, the liquor coming to the evaporator is not superheated and cannot therefore promote the intensification of the pulp circulation in the evaporator.